How deep should a mantel be?

I'm curious about something related to interior design, specifically about mantels. Could you please enlighten me on the appropriate depth for a mantel? I've seen various designs and depths, but I'm not quite sure which one would be ideal for my fireplace. Is there a standard depth that most designers adhere to? Or does it vary depending on the size and style of the fireplace? I'd appreciate any insights you could provide on this matter.